Improved support for heterogeneous cellular communication network operations is part of the ongoing specification of a Long Term Evolution (LTE) communication system by the Third Generation Partnership Project (3GPP) in its Release 10 Technical Specifications (TS) and later Releases. 3GPP technical specifications for LTE networks can be seen as an evolution of the technical specifications for current wideband code division multiple access (WCDMA) networks. An LTE network is sometimes also called an Evolved Universal Terrestrial Radio Access (E-UTRA) Network (E-UTRAN). A heterogeneous communication network has a mixture of cells of differently sized and overlapping coverage areas.
FIG. 1 depicts an example of a heterogeneous network 100 that includes three non-overlapping pico cells 110, 112, 114 deployed within the coverage area of a macro cell 120. It will be understood that the network 100 typically includes more than one macro cell 120, each of which can have zero, one, or more pico cells. Examples of pico cells and similar low-power nodes in heterogeneous networks are home base stations and relay nodes. As discussed below, the large difference in transmitted output power between a macro cell (e.g., +46 dBm) and a pico cell (e.g., less than +30 dBm) results in an interference situation different from that seen in networks in which all cells, or radio access network (RAN) nodes, have the same output power. In an LTE network, RAN nodes are generally called evolved NodeBs (eNodeBs, or eNBs).
Aims of deploying low-power nodes such as pico base stations within a macro cell's coverage area are to improve system capacity by cell splitting gains and to provide users with wide area experience of high speed data access throughout the network. Heterogeneous network deployments can be particularly effective by covering traffic hotspots, i.e., small geographical areas with high user densities, with pico cells, and they represent an alternative to deploying a denser macro-cell network.
A simple way to operate a heterogeneous network is to use different radio frequencies in the network's different layers to avoid radio interference between the layers. Thus in the network depicted by FIG. 1, the macro cell 120 and pico cells 110, 112, 114 would use different, non-overlapping carrier frequencies. With no macro-cell interference toward the underlying pico cell(s), cell splitting gains are achieved when all radio resources can simultaneously be used by all underlying pico cells. Nevertheless, a drawback of having network layers use different carrier frequencies is that it can lead to inefficient resource utilization. For example, when resource usages in the pico cells are low, it can be more efficient to use all carrier frequencies in the macro cell, thereby essentially switching off the pico cells. This is not typically possible, however, because the allocation of carrier frequencies across layers is typically fixed.
Another (related) way to operate a heterogeneous network is to share radio resources on a carrier by coordinating transmissions across the network layers, which is to say, by coordinating transmissions in the macro cell and underlying pico cells. This can be called inter-cell interference coordination (ICIC), in which certain radio resources are allocated to a macro cell during certain time periods, thereby enabling remaining radio resources to be used by underlying pico cell(s) without interference from the macro cell. This kind of resource sharing can change over time to accommodate different traffic demands and traffic situations across the network layers. In contrast to fixed use of different carrier frequencies, ICIC resource sharing can be more or less dynamic, depending on the implementation of the interface between the cells, or network nodes.
In an LTE network, for example, eNBs can communicate with each other via an X2 interface, and so an eNB can readily inform other eNBs that it will reduce its transmit power on certain resources. Time synchronization of the eNBs is required to ensure that ICIC works efficiently, and this is particularly important for time-domain-based ICIC schemes, in which radio resources are shared in time on the same carrier.
LTE uses orthogonal frequency division multiplex (OFDM) in the downlink (DL) from an eNB to user equipments (UEs), or terminals, in its cell, and discrete Fourier transform (DFT)-spread OFDM in the uplink (UL) from a UE to an eNB. LTE communication channels are described in 3GPP Technical Specification (TS) 36.211 V9.1.0, Physical Channels and Modulation (Release 9) (December 2009) and other specifications. For example, control information exchanged by eNBs and UEs is conveyed by physical uplink control channels (PUCCHs) and by physical downlink control channels (PDCCHs).
FIG. 2 depicts the basic LTE DL physical resource as a time-frequency grid of resource elements (REs), in which each RE spans one OFDM subcarrier (frequency domain) for one OFDM symbol (time domain). The subcarriers, or tones, are typically spaced apart by fifteen kilohertz (kHz). In an Evolved Multicast Broadcast Multimedia Services (MBMS) Single Frequency Network (MBSFN), the subcarriers are spaced apart by either 15 kHz or 7.5 kHz. A data stream to be transmitted is portioned among a number of the subcarriers that are transmitted in parallel. Different groups of subcarriers can be used at different times for different purposes and different users.
FIG. 3 generally depicts the organization over time of an LTE DL OFDM carrier in the frequency division duplex (FDD) mode of LTE according to 3GPP TS 36.211. The DL OFDM carrier comprises a plurality of subcarriers within its bandwidth as depicted in FIG. 2, and is organized into successive frames of 10 milliseconds (ms) duration. Each frame is divided into ten successive subframes, and each subframe is divided into two successive time slots of 0.5 ms. Each slot typically includes either six or seven OFDM symbols, depending on whether the symbols include long (extended) or short (normal) cyclic prefixes.
FIG. 4 also generally depicts the LTE DL physical resource in terms of physical resource blocks (PRBs, or RBs), with each RB corresponding to one slot in the time domain and twelve 15-kHz subcarriers in the frequency domain. Resource blocks are consecutively numbered within the bandwidth of an OFDM carrier, starting with 0 at one end of the system bandwidth. Two consecutive (in time) resource blocks represent a resource block pair and correspond to two time slots (one subframe, or 0.5 ms).
Transmissions in LTE are dynamically scheduled in each subframe, and scheduling operates on the time interval of a subframe. An eNB transmits assignments/grants to certain UEs via a PDCCH, which is carried by the first 1, 2, 3, or 4 OFDM symbol(s) in each subframe and spans over the whole system bandwidth. A UE that has decoded the control information carried by a PDCCH knows which resource elements in the subframe contain data aimed for the UE. In the example depicted by FIG. 4, the PDCCHs occupy just the first symbol of three symbols in a control region of the first RB. In this particular case, therefore, the second and third symbols in the control region can be used for data.
The length of the control region, which can vary from subframe to subframe, is signaled to the UEs through a physical control format indicator channel (PCFICH), which is transmitted within the control region at locations known by the UEs. After a UE has decoded the PCFICH, it knows the size of the control region and in which OFDM symbol data transmission starts. Also transmitted in the control region is a physical hybrid automatic repeat request (ARQ) indicator channel (PHICH), which carries acknowledged/not-acknowledged (ACK/NACK) responses by an eNB to granted uplink transmission by a UE that inform the UE about whether its uplink data transmission in a previous subframe was successfully decoded by the eNB or not.
Coherent demodulation of received data requires estimation of the radio channel, which is facilitated by transmitting reference symbols (RS), i.e., symbols known by the receiver. Acquisition of channel state information (CSI) at the transmitter or the receiver is important to proper implementation of multi-antenna techniques. In LTE, an eNB transmits cell-specific reference symbols (CRS) in all DL subframes on known subcarriers in the OFDM frequency-vs.-time grid. CRS are described in, for example, Clauses 6.10 and 6.11 of 3GPP TS 36.211. A UE uses its received versions of the CRS to estimate characteristics, such as the impulse response, of its DL channel. The UE can then use the estimated channel matrix (CSI) for coherent demodulation of the received DL signal, for channel quality measurements to support link adaptation, and for other purposes. LTE also supports UE-specific reference symbols for assisting channel estimation at eNBs.
Before an LTE UE can communicate with the LTE network, i.e., with an eNB, the UE has to find and synchronize itself to a cell (i.e., an eNB) in the network, to receive and decode the information needed to communicate with and operate properly within the cell, and to access the cell by a so-called random-access procedure. The first of these steps, finding a cell and syncing to it, is commonly called cell search.
Cell search is carried out when a UE powers up or initially accesses a network, and is also performed in support of UE mobility. Thus, even after a UE has found and acquired a cell, which can be called its serving cell, the UE continually searches for, synchronizes to, and estimates the reception quality of signals from cells neighboring its serving cell. The reception qualities of the neighbor cells, in relation to the reception quality of the serving cell, are evaluated in order to determine whether a handover (for a UE in Connected mode) or a cell re-selection (for a UE in Idle mode) should be carried out. For a UE in Connected mode, the handover decision is taken by the network based on reports of DL signal measurements provided by the UE. Examples of such measurements are reference signal received power (RSRP) and reference signal received quality (RSRQ).
Depending on how the measurements, possibly complemented by a configurable offset, are used, the UE can be connected to the eNB having the strongest received power, or to an eNB having the best path gain (lowest path loss), or a combination. Those do not usually result in the same selected cell, as the output powers of eNBs of different types can be different. This is sometimes called link imbalance. For example, the output power of a pico cell or a relay node can be on the order of more than 16 dB less than the output power of an overlying macro cell. Consequently, even for a UE that is close to a pico cell, the downlink signal strength from an overlying macro cell can be larger than that of the pico cell. From a downlink perspective, it is better to select a cell based on downlink received power, but from an uplink perspective, it is better to select a cell based on the path loss.
FIG. 5 depicts that cell selection dilemma for the macro cell 120 and underlying pico cell 114, showing an UL cell border that is closer to the macro cell 120 than is the DL cell border. The UL border is the locus of points yielding the same UL signal level at the cells 114, 120, and the DL border is the locus of points with the same DL signal level from the cells 114, 120. The UL signal levels are related to the path losses between a UE and the respective cells, and the inverse path loss levels (path gains) are indicated by dashed lines. The received DL signal levels are indicated by solid lines. The UL border is closer to the macro cell 120 than is the DL border due to the higher path loss to the macro cell. A UE disposed between the UL and DL borders could select the macro cell 120 based on its higher received power, or it could select the pico cell 114 based on its higher path gain (lower path loss).
In the scenario depicted by FIG. 5, it might be better from a network perspective for a UE between the UL and DL borders to connect to the pico cell 114 even when the UE's received signal level of the macro cell downlink is 10-12 dB (or even more) stronger than the UE's received signal level of the pico cell downlink. Such operation requires ICIC across the network layers when UEs operate within the region between the UL border and the DL border (i.e., “the link imbalance zone”).
FIG. 6 illustrates a simple-minded way to provide ICIC across network layers. In FIG. 6, successions of DL subframes transmitted by a macro 120 interfere with subframes transmitted by a pico cell 114, but the macro cell alternates typical subframes 610 and subframes 620 in which it does not transmit PDCCHs (indicated by the areas at the beginnings of the subframes) or data. The pico cell 114 is made aware of the regular locations (times) of the “blank” subframes 620, which cause low interference toward the pico cell. The pico cell 114 can schedule communications with its UEs operating in the link imbalance zone in subframes aligned with the macro cell's blank subframes 620.
The arrangement of regularly occurring “blank” subframes depicted in FIG. 6 is not practical for implementing ICIC and reducing interference across layers of a heterogeneous network for several reasons. Cell search is more difficult because a UE might make its downlink measurements in a cell's “blank” subframes, the positions of which the UE typically cannot know until it has synchronized itself to the cell. In addition, LTE legacy UEs, such as UEs complying with older versions of the 3GPP specifications, need CRS to be transmitted in all subframes, even the regularly occurring “blank” subframes 620, and so the subframes 620 cannot be completely blank. It is also difficult to avoid transmitting other DL channels in the “blank” subframes. For example, to avoid PHICH transmissions in the blank” subframes 620, it would be necessary to preserve the HARQ timing specified by 3GPP LTE Release 8, in which a normal FDD HARQ subframe always occurs 8 ms after the subframe carrying a UE's uplink grant. Simply not transmitting the PHICH is not a practical option, as that would likely result in uplink re-transmissions.
Moreover, the 3GPP LTE Release 8 and Release 9 specifications do not specify a set of subframes a UE is to use for DL signal measurements (that is left to the UE manufacturers for implementation), and those specifications do not support signaling to legacy UEs for requesting measurements in only certain subframes. Thus, in a network of time-synchronized macro cells, legacy terminals periodically making DL measurements can find themselves always performing their measurements in “blank” subframes 620, with the result that the measurements do not accurately reflect the interference situation in (possibly non-blank) subframes where the legacy terminals are scheduled. This can result in increased numbers of re-transmissions and degraded system performance, including dropped calls due to terminals' not being handed over to other cells at the appropriate times.